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  • 1.  Continuous Drop Panels

    Posted 10-27-2020 11:49 AM
    I am designing a low-rise concrete flat slab building.  The lateral system is an intermediate moment frame incorporating the slab and drops.  In order to meet the seismic drift limit, we have made the drop panels continuous in the building's weaker direction.  ACI 318-14 Chapter 18 has very different design and detailing requirements for beams than for slabs with drop panels.  Is it appropriate to design continuous drop panels (much wider than the columns) as beams, and if so, under what criteria?  Are there geometry limits that would preclude considering drop panels as beams?

    For everyone's benefit, I would love to hear responses general to all flat slab systems with continuous drops, not specific to my project.

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    Christian Parker EIT, A.M.ASCE
    Structural Project Engineer
    Washington DC
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  • 2.  RE: Continuous Drop Panels

    Posted 10-28-2020 02:47 PM
      |   view attached
    Dear Christian, you have an interesting problem. Enclosed, you will find a document from Structure Point that could help you a lot dimensioning the flat slab with drop panels.
    From my point of view,  continuous drop panels for seismic purposes could include rebar for flexural strength as a beam (as an immersed beam in the continuous drop panel).

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    Andres Guzman Ing., M.ASCE
    Associate Professor
    UNIVERSIDAD DEL NORTE
    Barranquilla
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    Original Message


  • 3.  RE: Continuous Drop Panels

    Posted 10-29-2020 02:35 PM
    Many thanks, Andres.  This document is a helpful reference.  Do I understand correctly that you are suggesting part but not all of the width can be considered as an effective beam?  If so, where could I find criteria for the width of the effective beam?

    For some more information, here are the (relatively manageable) shear design requirements for beams in an IMF:
    Here are the (very challenging to satisfy) shear requirements for flat slabs with or without drops in an IMF:
    0.4PhiVc!  That's like using an effective Phi factor of 0.3 for punching!  The escape clause at the end forwards to the designer's choice of a very stringent drift limit or a high reinforcement requirement which is almost impossible to satisfy for drop panels.

    So the question, in addition to the flexural demands on the drop panels as part of the lateral system, is which provisions for shear design are applicable to continuous drop panels in an intermediate moment frame?


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    Christian Parker EIT, A.M.ASCE
    Structural Project Engineer
    Washington DC
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    Original Message


  • 4.  RE: Continuous Drop Panels

    Posted 11-01-2020 12:27 PM
    Dear Christian, you are right, just a part but not all of the width could be considered as an effective beam, regarding reinforcement (as beams, in a moment frame). Nevertheless, please read the lines and suggestions of Mandeep (you should use a structural system for vertical loads and assign responsibilities to other elements for lateral loads (e.g. shear walls, concrete bracing/diagonals). Please let us know how do you accomplish the drift's requirements.

    Regarding provisions for shear design, I suggest you consider the unpredictable behaviour of shear and also, the concrete cracking sections. Shear requirements could be checked by FEM for the most critical section.

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    Andres Guzman Ing., M.ASCE
    Associate Professor
    UNIVERSIDAD DEL NORTE
    Barranquilla
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    Original Message


  • 5.  RE: Continuous Drop Panels

    Posted 11-02-2020 07:56 AM
    Chiming In!
    The situation is mainly the same situation for the engineer to develop the stress in a plate scenario.  So, by accessing the platesandshells set up your limits in applying your finite element model then the thickness of your flat slab acts as a beam within the modes with which the stresses develop.  Depict the summation of stresses that create your delimiting stress regime.  And detail it as though detailing a beam.  When a hoop tie cannot fit in the slab, use stud rails.  I hope it helps.

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    Refugio Rochin P.E., M.ASCE
    Asst. Eng.1
    Naxutl, Inc.
    Albuquerque NM
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    Original Message


  • 6.  RE: Continuous Drop Panels

    Posted 10-30-2020 08:18 AM
    ​

    Hi Christian,

    Flat slab buildings have very low lateral stiffness and hence they are prone to large deflections even for very small levels of shaking. This excessive drift can further amplify the secondary moments on the columns due to P-Delta Effect. This is the reason ACI does not allow the use of this type of construction in high seismic regions and even forbids it use for some SDC.

    Regarding the effective width calculation and other aspects of the design of flat slabs you may refer the ACI-ASCE joint publication "GUIDE TO DESIGN OF REINFORCED TWO-WAY SLAB SYSTEMS (ACI 421.3R-15)". Section 7.2 provides a method of calculating the effective width of the slab for lateral load purpose and also provides a method for designing these for seismic/ Lateral load.

    However do read the below extract from the same specification. I would suggest you add a secondary lateral load system by terms of shear wall etc, and use the slab column system just for gravity loads.

    A slab-column structure acting as the seismic-force-resisting system is likely to be far too flexible for higher seismic design categories (SDCs) and would not deliver performance consistent with other requirements.
    For earthquake ground motions, slab-column framing systems designed according to Chapters 1 through 6 can be used in ordinary moment frames, and are appropriate for SDC A or B. If a slab-column frame meets additional requirements described in this chapter, it may be considered as an intermediate moment frame and is allowed for structures assigned to SDC C (ASCE 7-10, Section 12.2). Additional requirements are related to the distribution of slab moments, arrangement of slab reinforcement, and punching shear related issues. For structures assigned to SDC D, E, or F, slab-column frames without beams are generally not permitted as part of seismic-force-resisting systems (ACI 318-14, Section R18.2), with limited exceptions under ASCE 7-10, Section 12.2. For example, one exception would be intermediate moment frames in a dual system with special reinforced concrete shear walls less than 100 ft (30.5 m) in height.
    The reasoning behind this is the slab-column frames cannot be detailed for the level of energy dissipation and ductility demanded for special moment frames. In the case of ductility demand, this chapter describes punching-shear related recommendations for slab-column connections, along with the design story drift estimated for the seismic force resisting system




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    Mandeep Singh Kohli CP, M.ASCE
    Senior Engineer
    India
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    Original Message


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